Hierarchical audio coding is dedicated to organizing bit streams resulting from audio coding in a hierarchical way, which are generally divided into one core layer and several extended layers. A decoder is able to implement to only decode the coded bit stream of a low layer (such as the core layer) in a situation of no coded bit stream of a high layer (such as a extended layer) available, and the more layers are decoded, the more the audio quality is improved.
The hierarchical coding technology has a very important practical value for a communication network. On one hand, data transfer can be completed by the cooperation of different channels, and packet loss rate of each channel may be different; and at this point, it often requires to perform a hierarchical process on the data, put important parts of the data into steady channels with relatively low packet loss rates for transmission, and put secondary parts of the data into non-steady channels with relatively high packet loss rates for transmission, so as to ensure that only a relative reduction of the audio quality occurs when the packet loss occurs in the non-steady channels, without a condition that one frame of data cannot be decoded completely. On the other hand, the bandwidth of some communications networks (such as Internet) is very unstable, and the bandwidths of different user terminals are various. It is impossible to use one fixed bit rate to meet the requirements from the users with different bandwidths, while the use of hierarchal coding scheme enables different users to obtain the respective optimum enjoyment regarding tone quality under their own bandwidth conditions.
Traditional hierarchical audio coding schemes, such as G.729.1 and G.VBR of the International Telecommunication Union (ITU), do not perform a targeted process for transient signal frames, and therefore, for signals comprising major transient components (such as a percussion signal), the coding efficiency is low, especially with moderate and low bit rates.